1. Field of the Invention
The present invention relates to a process for continuously crystallizing a high purity adduct of bisphenol A with phenol.
More particularly, the invention relates to a process for continuously crystallizing the high purity adduct of bisphenol A with phenol from a slurry having the highest concentration which can continuously separate the adduct crystal and filtrate from the slurry by controlling the amount fed to a crystallizer with a computer.
2. Description of the Related Art
There has recently Been an increased demand for bisphenol A as a material for engineering plastics in addition to a material for polycarbonate resin and epoxy resin. Colorless and high purity bisphenol A is required for these uses.
In a process for preparing the high purity bisphenol A, for example, phenol is reacted with acetone in the presence of an acid catalyst, and the reaction product is treated to remove the catalyst, water and a small amount of phenol. The residual liquid mixture is cooled to crystallize bisphenol A in the form of an adduct with phenol, precipitated adduct crystals are separated from the mother liquor, and phenol is removed from the adduct to recover bisphenol A.
The process of preparing high purity bisphenol A by crystallizing the adduct of bisphenol A with phenol (hereinafter referred to as bisphenol A-phenol adduct) has been disclosed, for example, in Japanese Patent Publication SHO 63-26735(1988). That is, a phenol solution of bisphenol A which was prepared in the presence of a hydrochloric acid catalyst is cooled to a range of 35.degree. to 70.degree. C. while feeding 2 to 20% by weight of water to the solution under reduced pressure, precipitated crystals of bisphenol A-phenol adduct are filtered to separate the crystals, and phenol is evaporated from the crystals to obtain high purity bisphenol A.
The process is characterized in that by-products and impurities are separated from bisphenol A by precipitating the bisphenol A-phenol adduct.
Precipitation of the adduct can be controlled by adjusting the amount of feed water so as to maintain a constant temperature under reduced pressure. However, in the process, a rapid increase in the concentration of bisphenol A in the phenol solution leads to a rapid increase in the concentration of the crystallized adduct in the slurry and causes problems such as blocking of the liquid transfer line to a slurry separation step.
Japanese Laid Open Patent HEI 1-213246(1989) discloses a process for crystallizing bisphenol A-phenol adduct from a phenol solution of bisphenol A wherein the concentration of bisphenol A in the solution is controlled by removing a portion of bisphenol A in the solution or by addition of phenol to the solution according to feed-back control on the basis of density measurement of the solution.
Further, Japanese Laid-Open Patent HEI 1-316335(1989) discloses a process for crystallizing bisphenol A-phenol adduct which is characterized by maintaining the temperature of the internal surface of the crystallizer at 5.degree. C. or less higher than the temperature of the reaction mixture in order to obtain the bisphenol A-phenol adduct in a high purity.
These methods are effective as long as the process is steadily carried out. However, the former cannot follow a rapid change of conditions and the latter does not disclose a practical procedure for controlling the crystallization.
A difficult and important technique to precipitate the crystals of bisphenol A-phenol adduct from a liquid mixture of bisphenol A and phenol is to detect operation conditions of the crystallization system which are always varying by disturbance and to maintain the concentration of the adduct in the slurry (hereinafter referred to as slurry concentration) at a constant level. As a practical problem for example, when the slurry concentration of the adduct rises above the desired level in the crystallizer, impurity containing filtrate attaches, to the crystal of bisphenol A-phenol adduct in the slurry separation step, or an increased amount of impurity is included into the crystal of the bisphenol A-phenol adduct, and hence purity and hue of the product bisphenol A are seriously damaged.
Further, too high a slurry concentration in the course of transferring the adduct slurry from the crystallizer to the slurry separation step leads to problems such as blocking of transfer piping in a short period and abrasion leaks or clogging of a pump in a medium or long period, and further causes troubles of the slurry separator itself. Thus, too high slurry concentration seriously impairs productivity and in the worst case, operation of the whole production system must be stopped.
On the other hand, when the slurry concentration of the crystallizer is set at a low level in order to avoid the above problems, the rate of crystallization remarkably decreases and scale up of crystallizer volume becomes necessary. Further, equipment for treating a large amount of filtrate is needed and thus efficiency of the whole production facility is severely decreased.
In order to solve these problems, it is important to prescribe a desired value for a slurry concentration and to carry out a rapid and accurate control of the slurry concentration. In order to accurately control the slurry concentration in the crystallizer, ever changing slurry concentration must be immediately checked. However, the slurry concentration depends upon a complex material balance and heat balance among the three phases of vapor, liquid and solid in the crystallizer, and is hence very difficult to manage and control.
The slurry concentration of the bisphenol A-phenol adduct becomes higher at a lower temperature and at a lower concentration of water. However, an increased amount of feed water to the crystallizer under reduced pressure leads to an increase in the slurry concentration due to the decrease in temperature caused by the latent heat of water evaporation, and contrary results occur when the slurry concentration is reduced by an increased amount of water due to non-evaporation of water. Thus, variation of the slurry concentration is difficultly detected due to the presence of both effects. Further, the system is complex because each phenomenon varies depending upon the concentration of bisphenol A and temperature of the solution. Therefore, rapid and accurate control of the slurry concentration is very difficult.
Accordingly, one object of the present invention is to provide a process for crystallizing a high purity bisphenol A-phenol adduct from the solution of bisphenol A in phenol without the above problems.